How to mirror a bone?
Blender 2.79b

I've just discovered that if you mirror with the x-Axis option in the tool box, then you have to name each bone manually for both sides.
If you are rigging a human, a humanoid character or an animal, the best way is NOT to check the x-Axis option but follow the steps below:


Edit Mode, select all the bones to mirror on one side (arm, leg,
hand fingers)
Make sure each bone name to be mirrored ends with .L which means LEFT (e.g.: hand.L). This is vital!
Make sure the cursor is centred (Shift + C).
Your mesh has to be centred as well so that the cursor is placed at the base of it
Press W and select Symmetrize (duplication is automatic)
Your bones are now mirrored and the names automatically updated with extention .R (which stands for right).


You are done!